Numerical investigation on the volute cutwater for pumps running in turbine mode

Abstract

The hydropower sector has recently raised the interest in pump as turbine (PaT) that can be a valid tradeoff between capital cost and performance in micro-scale installations. Nevertheless, the modest efficiency of PaTs often restricts their exploitation. In this paper, available experimental data are used to tune a numerical model which aims to investigate the effect of the pump cutwater design on the PaT performance to improve the hydraulic efficiency. Because of its finite thickness, the cutwater interferes with the flow at the runner inlet, and generates local flaws in the velocity field such as swirl and deviations of the streamlines which limit the machine performance. To identify the geometrical characteristics of the cutwater impacting on the PaT performances, different values of stretching and thickness of the cutwater are studied at variable inclination by computational fluid dynamics (CFD) simulations. A multivariate regression method is applied on the CFD results to build a surrogate model of the PaT hydraulic characteristics as a function of the geometrical parameters of the machine cutwater. Based on this model, an optimization problem is solved to identify the most advantageous geometrical asset of the PaT cutwater to maximize the efficiency. The results highlight that the length and the cutwater angle are the most affecting variables in favouring a tangential component at the entrance of the runner. The hydraulic efficiency peak of the optimized geometry results to be 86.3%, while the baseline configuration records an efficiency of 82.4%, and the J À f characteristic moves the best efficiency point towards higher head (þ7.5%) and lower discharge (À13.0%). The proposed methodology allows identifying the best geometrical characteristics of the PaT cutwater to maximize the performances while significantly reducing the computational time.

Prediction method of centrifugal pump running in turbine mode based on losses analysis

“…Nevertheless, CFD data are showing discrepancies compared to experimental data. CFD versus experimental data discrepancies may come from geometry simplification performed in CFD studies [38] . In addition to the above, accuracy of CFD methods depends on parameters such as mesh quality, turbulence model and boundary conditions [39] , [40] , [41] .…”

Effect of blade layout on performance of low specific speed pump operating in turbine mode

“…The throat area and shape of volute cutwater (or so called volute tongue) are of great importance for PaT operation since the flow angle given by the volute must be processed at the impeller inlet [11]. For the pump investigated in this study the flow angle given by the volute throat area is estimated to be α 3 = 16°, which is the average value between upper wall angle (23°) and lower wall angle (9°) of throat area, see figure 3.…”

A Numerical Investigation into the PAT Hydrodynamic Response to Impeller Rotational Speed Variation

“…As expected, unsteady flow phenomena such as the backflows, shock flows, and vortical flows, have inflicted higher entropy and energy dissipation, where flow zones with the highest energy losses have been pointed out as the vicinities of the blade leading edge, the cavity zone (impeller's back and front chambers), and volute throat. In the same respect, Guan et al [31] and Morabito et al [32] found that the position and geometry of the volute throat may influence the machine energy loss characteristics and its efficiency.…”